omit environmental nodes from enrichment/depletion analysis

[hariszaf]

@ermismd
@hallucigenia-sparsa

Environmental nodes should be omitted from the total nodes in a cluster before the enrichment/depletion analysis is carried out.

To make this easier, we may exploit the microbetag::taxonomy namespace and assign that to env_var or something similar in the back-end.

Then, in the front-end, you just exclude from the analysis nodes with a microbetag::taxonomy of this value.

Also, this way it is gonna be super easy to have a different shape for these nodes, i.e. editing the visual style.